In Long Term Evolution (LTE) Release 8 (Rel-8) specifications, Multi User Multiple Input, Multiple Output (MU-MIMO) transmission is supported in downlink transmission by using transmission mode 5 in the physical layer. If MU-MIMO is specified in such transmission mode, a User Equipment (UE) will feedback a Precoding Matrix Indicator (PMI) and Channel Quality Indicator (CQI) to an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) Node B (eNB) and the eNB will schedule two or more UEs together and signal to the UEs the precoding matrices used for transmission. The transmit power to each UE may then be properly scaled to maintain a constant total transmit power and such power scaling factor may also be signaled to the UE.
The UE will use a Common Reference Signal (CRS) for channel estimation. Thus, other than the power scaling, the MU-MIMO scheme under Rel-8 is almost the same as a closed loop Single User MIMO (SU-MIMO) scheme without any special treatment for MU-MIMO.
In LTE Advanced (LTE-A), various features are being considered. Among them are that the reference signal (RS) is defined into two categories, one for Channel Measurement (CSI-RS) and the other for Demodulation (DM-RS). This is different from the Rel-8 specifications, where channel estimation and demodulation all use the same set of common reference signals, the CRS. Furthermore, the DM-RS should be pre-coded in the same way as for data, making the RS a Dedicated Reference Signal (DRS).
In LTE Rel-9, a work item being investigated is the performance of a dual layer beamforming technique. In such a system, two independent data streams are encoded, modulated and mapped to frequency resources. The data streams are then transmitted on two independent beams from a set of antennas, a subset of which may have low mutual correlation. For example the set of antennas could be an array of half wavelength spaced dual polarized elements or the set could be two panels separated by 4 or more wavelengths, where each panel contains half wavelength spaced elements. DRS is also used for demodulation.
This use of a Dedicated Reference Signal creates problems with regard to control signaling. Efficiency is one design consideration for control channels, since control channel overhead impacts system capacity.
Efficient control signaling schemes have been developed in the area of resource allocation. In particular, in order to allocate one or more of a plurality of radio resources, several signaling schemes have been developed. For example, if there are N radio resources, then a bitmap of length N, where each bit represents one radio resource, can be used to indicate a resource allocation. Alternatively, if there are N radio resources, then a first signaling field can be used to indicate the first radio resource in a resource allocation and a second signaling field can be used to indicate the number of radio resources in the allocation. Efficient signaling is also desirable for DRS.